Polyethylene is divided into high-density (HDPE, density 0.941 g/cc or greater), medium-density (MDPE, density from 0.926 to 0.940 g/cc), low-density (LDPE, density from 0.910 to 0.925 g/cc), and linear low-density polyethylene (LLDPE, density from 0.910 to 0.925 g/cc). (See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials.)
One of the main uses of polyethylene (HDPE, LLDPE, and LDPE) is in film applications, such as grocery sacks, institutional and consumer can liners, merchandise bags, shipping sacks, food packaging films, multi-wall bag, liners, produce bags, deli wraps, stretch wraps, and shrink wraps. The key physical properties of polyethylene film include tear strength, impact strength, tensile strength, stiffness and transparency. Overall film strength is desirable so that the films may be employed with confidence in various applications. Film transparency is desirable because a transparent film allows the users to see inside the bag without opening it. While HDPE films have superior strength, toughness and puncture resistance, they have low transparency and low gloss.
Machine direction orientation (MDO) is known to the polyolefin industry. When a polymer is strained under uniaxial stress, the orientation becomes aligned in the direction of pull. Most commercial MDO films are produced by orienting cast extrusion films. When an HDPE film undergoes MDO, the resultant film usually shows improved gloss and transparency. However, the improvement is very limited and the MDO films remain opaque.
Other ideas to improve film gloss and transparency are also known. For instance, U.S. Pat. No. 5,989,725 teaches a multilayered film of which at least one layer is an HDPE. The multilayered film shows increased transparency and gloss. However, the multilayered film is usually more costly than a single-layer film. Further, the claimed multilayered film still has a high haze and low gloss compared to LLPDE films. Similarly, European Patent Application EP 0246328 teaches a transparent film made from a blend of HDPE and LLDPE.
It would be desirable to prepare a polyethylene film which has a physical strength like HDPE film but transparency and gloss like LLDPE film. It is more desirable to prepare a transparent polyethylene film without using multiple layers or blending two or more different resins.